Optical isolators are well-understood in the art and find uses in optical communication systems, sensors and the like. The purpose of an optical isolator is to eliminate unwanted or reflected optical signals from interfering with a desired optical function. For example, an isolator may be inserted in an optical signal path between a distributed feedback (DFB) laser and an optical fiber. Without the isolator, unwanted optical signals (i.e., reflections) from the optical fiber would couple back into the DFB laser and adversely affect its transmitted optical spectrum. By including an isolator in this design, unwanted reflected signals are absorbed by the isolator and do not reach the laser. Non-planar optical isolators typically employ birefringent crystal plates (e.g., rutiles), half-wave plates and latching garnets or non-latching garnets with external magnets (hereinafter referred to in general as “Faraday isolators”).
A planar, waveguide-based isolator has been developed in the optical domain for integration with a semiconductor light emitting diode, such as Fabry-Perot or ring laser diodes, as disclosed in U.S. Pat. No. 5,463,705, issued to R. Clauberg et al. on Oct. 31, 1995. In the Clauberg et al. arrangement, a waveguide directional coupler is formed within one layer of a III–V material system (e.g., GaAs, InP, etc.) so as to be integrated with a III–V based light emitting diode. The directional coupler is formed as a rib waveguide directional coupling structure, where at least one branch of the coupler includes an “absorber means” to collect a reflected, unwanted optical signal and reduce its further propagation through the coupler (i.e., “isolates” the reflected signal). While the Clauberg et al. waveguide coupler including at least one isolating branch may be integrated with a III–V light emitted device to increase optical efficiency, the emitter/isolator remains as a discrete component that must ultimately be combined with various other optical and electronic elements that are not well-suited to formation in III–V materials.
Indeed, as the complexity of optical system design increases, the need to monolithically integrate multiple optical and electrical functions onto a single material substrate is becoming a necessity, in order to reduce the size and cost of the optical system. It has recently been recognized that the materials, processes and fabrication techniques used in the production of silicon-based electronic devices can be adapted for the processing of optical elements. Advantageously, it has now been shown that various optical elements and their associated electronic activation devices can be integrated within the same substrate. The base materials system of choice for this simultaneous integration of electronics and optics is silicon-on-insulator (SOI), where the electronics have been well-characterized for many decades using CMOS processing technology, and the optics can be reduced to extremely small sizes as a result of the inherently high index optical guides that can be fabricated in SOI.
The monolithic integration of a conventional Faraday isolator into a planar silicon integrated circuit fabrication process is not considered to be practical. Indeed, the conventional Faraday isolator device relies on the use of the magneto-optic effect to rotate the light and provide isolation. Silicon does not exhibit any such magneto-optic effect. Various materials that do exhibit a magneto-optic effect are not considered to be compatible with conventional silicon semiconductor processing technology.
Thus, a need remains in the art for an optical isolator structure that is compatible with the silicon processing techniques used in the formation of SOI-based optoelectronic devices.